In suppression of torque ripple with a periodic disturbance observer, adjustment of a gain portion is determined sequentially during monitoring operation of monitoring an operating condition. Therefore, many adjusting parameters are involved and achievement of correction is dependent on the skill of a person in charge of adjustment or design. The periodic disturbance observer is provided with a model correcting means or section which calculates an error of an identification model by using a time difference quantity by an output of the periodic disturbance observer and a sensed value of a plant and corrects the identification model with the error of the identification model. Finally, the system performs the torque ripple suppression control with an accurately estimated periodic disturbance.